• Griffin, Ralph H.
  Life Sciences and Agriculture Experiment Station, University of Maine at Orono
  
• Johnson, James E.
  Life Sciences and Agriculture Experiment Station, University of Maine at Orono
  

Fifty-five variable sized sample plots, each sufficient in area to encompass ten spruce and/or balsam fir occupying overstory positions in the canopy, were established in even-aged spruce-fir stands varying  in age, stand density, and site characteristics in northern Maine. Height age data pairs obtained from stem analyses of the six tallest trees on each sample plot were graphed to determine the height of the tallest spruce and/or the tallest balsam fir at each five-year-ago interval of stand development above breast height. The height-age data pairs for spruce and balsam fir were stratified respectively into seven five-foot site index classes, (base age 50 at breast height) 35 through 65. Data for each site index class were fitted into the equation (Age)2/Height=b0+b1(Age) + b2(Age)2, to obtain initial values for the b0 coefficients. The relationship of site index to each of the b0 coefficients was analyzed by linear regression. The basic equation was then expanded to include site index as an independent variable. Regression analysis was used to formulate equations defining polymorphic site index curves for spruce and balsam fir respectively growing in even-aged stands in northern Maine.